Scientific processes often require accurate volume measurements of liquids. Highly accurate volume measurements are particularly important in chromatography. In gel permeation and hydrodynamic chromatography, the cumulative volume of eluent fluid is the key parameter for calculations used to determine the components of the sample being analyzed. In liquid chromatography, the flow rate of eluent through the system must be known at all times to allow accurate interpretation of results, and the most accurate way to determine the flow rate is by using accurate volume and time measurements.
Prior art volume measurement techniques have generally been inconvenient and error prone. Typically, the flowing liquid is collected in a previously empty container of known dimensions, such as a measuring cylinder, pipette, or volumetric flask. This method is limited by the size of the container and is labor intensive if volume accumulation must be recorded at short intervals of time. Floats have been provided in such containers and the rise of the float recorded, but such a system will not suffice for long term accumulations of liquid.
Another method is based on weighing the accumulating liquid, but this requires a knowledge of the density of the liquid. In chromatography, the eluent may be a mixture of two or more chemicals whose mix ratio is altered during the course of an experiment, so that the density and viscosity of the eluent are continuously changing, making accurate volume calculations based on the weight of the accumulated liquid impossible. Changes in density and viscosity also affect the accuracy of flow meters using rotors inserted into the flowing liquid.
Other volume measurement methods include the injection of bubbles of air or heat pulses into the liquid flow. The injections are then timed as they pass between two points. In such a method, short term volume and flow deviations are not detected. Heat pulses cannot be used if the heat would affect the chemical eluent, and in slow flow situations, the heat pulse dissipates and cannot be accurately located. Even under optimum conditions, the bubble or heat pulse cannot always be located with the precision required.
Another volume measurement device is known as a syphon counter. The eluent is accumulated in a U-shaped trap, and when the trap fills, siphon action drains the trap as fast as the liquid viscosity will allow. The departing liquid passes a sensor which adds the known volume of siphoned liquid (typically one milliliter) to the accumulated volume. The time between each discharge allows determination of the flow rate. Accuracy of this system is not sufficient for some forms of chromatography, and the system cannot be used to measure microliter volumes.